A complex material in which inorganic compound particles are dispersed into a matrix material such as solvent, resin, glass, or ceramic may be provided with properties such as an optical property, a thermal property, a mechanical property, an electrical property, and a magnetic property, those of which may not be achieved by the matrix material alone. Therefore, the complex material has been put to practical use for various applications.
In particular, when inorganic nanoparticles are used, a specific function or property attributed to the size thereof may be imparted, and hence, the inorganic nanoparticles have been attracting attention in various fields.
For example, in the case where inorganic nanoparticles having high refractive index are uniformly dispersed into a matrix material, visible light is scattered and the resultant looks clouded when nanoparticles each having a particle diameter larger than several tens of nanometers are used, but it is expected that an optical material having high transparency and high refractive index with suppressed light scattering may be obtained by using nanoparticles each having a particle diameter smaller than several tens of nanometers. Consequently, the inorganic nanoparticles and an optical material obtained by dispersing the inorganic nanoparticles into a film or a lens have been studied as a material whose light scattering, refractive index, and the like are adjustable.
In a practical sense, however, the nanoparticles are extremely unstable when used alone, and easily aggregate to enlarge the particle diameters thereof even when the surface is covered with various substances, and hence, it has been difficult to realize a nanoparticle/matrix-material complex for the optical applications.
As a way to avoid the above problem, there is described, in P. D. Cozzoli et al., J. Am. Chem. Soc., 125, 14539-14548 (2003), a method of producing metal oxide nanoparticles by an in-situ surface modification nanoparticle synthesis, which involves performing surface modification of a metal oxide simultaneously with crystallization of the metal oxide. However, although the nanoparticles of several nanometers and the dispersion liquid thereof has been produced, a colorless transparent nanoparticle dispersion liquid has not been produced at high concentrations of several tens of wt %.
Further, as a high-concentration nanoparticle dispersion liquid, there is described in Japanese Patent Application Laid-Open No. 2007-254245 a high-concentration water-dispersion liquid of metal oxide nanoparticles, the nanoparticles being covered with a dispersant having a heterocyclic type cationic group. In Japanese Patent Application Laid-Open No. 2007-254245, however, although there is described that the nanoparticle dispersion liquid is excellent in dispersibility and long-term stability in water, there is no description on the dispersibility and the long-term stability of the nanoparticle dispersion liquid in an organic solvent or in a polymer. Further, it may be presumed that a nanoparticle dispersion liquid at high concentrations equal to or more than several tens of wt % is insufficient in its transparency and suppression of coloring. In addition, the method according to Japanese Patent Application Laid-Open No. 2007-254245 involves dispersing commercially-available nanoparticles into water by being covered with a dispersant, and hence, an increase in particle diameter due to the aggregation during the production process may not be avoided.